Field of the Invention
The present invention relates to a contactless power transfer system for transferring electric power from a power transmission device to a power reception device in a contactless manner.
Description of the Background Art
Japanese Patent Laying-Open No. 2013-215066 discloses a power transfer system for transferring electric power from a power transmission device to a power reception device in a contactless manner. In this power transfer system, the power reception device includes a power receiving-side resonator, a rectifier, a DC/DC converter, and a control device. Based on the input voltage of the DC/DC converter, the control device calculates a current command value at which the input impedance of the DC/DC converter is set at a set value, and then, controls the DC/DC converter such that the input current of the DC/DC converter becomes equal to the current command value.
According to this power transfer system, since the input impedance of the DC/DC converter is maintained constant during the operation period of the DC/DC converter, input impedance matching can always be achieved, so that power transfer efficiency can be improved (see Japanese Patent Laying-Open No. 2013-215066).
In the above-described power transfer system, the magnitude of the coupling coefficient between the power transmission device and the power reception device is not taken into consideration. In the case where the DC/DC converter is controlled so as to have an input impedance kept constant, if the coupling coefficient is relatively small, the power transmission voltage needs to be lowered in the power transmission device in order to ensure the desired power reception current (the input current of the DC/DC converter). In the case where prescribed electric power is transferred from the power transmission device to the power reception device, when a voltage is lowered in the power transmission device, the flowing current is increased. Consequently, loss is increased, so that the efficiency may entirely decrease.